Liquid based boiler

ABSTRACT

Methods and systems generate steam for oil recovery operations. The systems may limit feedwater pretreatment expenses and fouling issues. In the method, dirty feedwater introduced into a vessel containing a hot liquid hydrocarbon, e.g., an already hot produced hydrocarbon, contacts the hydrocarbon and is vaporized into steam. The steam collects in a top of the vessel and may be conveyed to the wellhead for downhole injection. The hydrocarbon remains heated by a closed circulation loop passing back and forth through a lower half of the vessel containing the hydrocarbon. The fluid in this loop remains isolated from contaminates in the water to limit fouling in tubes, which form the loop and can employ normal metallurgy to save on capital costs. The hydrocarbon can be treated as needed to remove accumulating salts and/or entrained water and recycled.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Patent Application Ser. No.61/983,742 filed Apr. 24, 2014 entitled “LIQUID BASED BOILER,” which ishereby incorporated by reference.

FIELD OF THE INVENTION

The invention relates to method and system for generating steam withminimal or eliminated fouling resulting largely from the use ofcontaminated feedwaters. The invention limits fouling problem byspraying dirty feedwater directly onto a hot hydrocarbon for steamgeneration.

BACKGROUND

Steam Assisted Gravity Drainage (SAGD) is an enhanced oil recoverytechnology for producing heavy crude oil and bitumen. It is an advancedform of steam stimulation wherein a pair of horizontal wells are drilledinto the oil reservoir, one a few meters above the other. High pressuresteam is continuously injected into the upper wellbore to heat the oiland reduce its viscosity, causing the heated oil and any condensed steam(hot water) to gravity drain into the lower wellbore, where it can bepumped to the surface. The produced oil is a mixture of heated oil pluswater. Because water is as precious a resource as oil, the “producedwater” is then cleaned and returned to the boiler, where it is convertedinto steam and injected back into the ground.

Due to the recycling of water in SAGD operations, and the fact that thewater encounters petroleum deposits as well as any additives used inproduction, the feedwater used to make steam is typically far from pure.Produced water and brackish well water are the main boiler feedwatersources for SAGD and other steam based oil recovery process. The waterat time of being generated into the steam may still contain: at leastabout 500 parts per million (ppm), at least 1000 ppm, at least 10,000ppm or at least 45,000 ppm total dissolved solids; at least 100 ppm, atleast 500 ppm, at least 1000 ppm or at least 15,000 ppm organiccompounds or organics; and at least 1000 ppm free oil.

“Fouling” is the contamination of heating surfaces by these mineralscales, and the build-up of scale eventually decreases the heat-flux andthus the heating efficiency. Therefore, the boiler has to be shut downseveral times a year to remove the fouling layer and/or repair thetubing. In addition to the repair cost, the down-time further increasesthe cost of the SAGD operation. To minimize fouling, boiler feed-water(BFW) quality is critical because dissolved solids are the major causeof boiler failure and efficiency losses. Therefore, the total dissolvedsolids (TDS) for BFW needs to be controlled under a certain level toprevent or alleviate the scaling issue, and this is usually done bypre-treating feedwater prior to use to reduce TDS.

The two most common types of steam generators used for oil sandsrecovery are once through steam generators (OTSG) and drum boilers,which are also called water tube boilers. Coal-fired steam generators,downhole steam generators, fluidized bed combustion boilers and vaportherm steam generators have previously been reported to be used inAlberta fields, but they are no longer found in recent fieldapplications.

The OTSG is a large continuous tube type steam generator wherein steamis produced at the outlet of the continuous tube, as shown in in FIG. 1.Feedwater supplied at one cold end of the tube undergoes thepreheating-evaporation cycle as it travels along the continuous tube. Assteam is produced in a traditional OTSG, the steam quality is usuallyaround 75-80%, i.e. not all the feedwater vaporizes.

In drum type steam generators, in contrast, preheated water evaporatesas it circulates in heated tubes between the steam drum and thefeedwater drum, as shown in FIG. 2. Saturated steam and water rises intothe steam drum due to the lowered density compared with the water indowncomer tube. Saturated steam is drawn off the top of the drum andsent to the superheater section.

OTSG systems require frequent cleaning, which leads to the increaseddown-time and costly repair. Fouling also reduces the thermal efficiency1% to 15% depending on the amount of deposits, as they act as aninsulating layer on the heating tubes. The shutdown to clean the scaleincreases operating costs, and the pre-treatments needed to de-oil andclean the feedwater before use also contributes significantly to cost.

Therefore, there is a need for an improved steam production scheme thatcan minimize fouling issues and reduce the downtime and reduce bothoperating and initial capital costs for SAGD and other steam based oilrecovery operations.

SUMMARY OF THE DISCLOSURE

Embodiments of the invention use a hot liquid, such as the producedheated hydrocarbons, or fractions thereof, to directly vaporizenon-treated boiler feedwater. This hot hydrocarbon receives its thermalenergy from another hot fluid, such as molten sodium, moltensodium-potassium, or another hydrocarbon that may include butane,DOWTHERM™ or THERMINOL™ heat transfer fluid, within coils in a closedcirculation loop traveling from a standard heater to the vesselcontaining the hot hydrocarbon. Contaminants from the water beingvaporized may thus buildup in the hot liquid requiring treatment of thehot liquid. The fluid in the coils transfers heat to the hot liquidwithout relying on transfer of the hot liquid to the heater. Thus, thefluid in the coils circulates to maintain a desired heat balanceproviding a benefit by enabling decoupled circulation of the hot liquidfor treatment, such as desalting, at a rate wanted for removal of thecontaminants independent of a flow needed for the heating.

The use of a hot hydrocarbon such as DOWTHERM™ enables more conventionalmetallurgies to be used for the coils, thus minimizing CAPEX costs.Further, the contaminants remain in the hot liquid outside the coilswithout passing to the heater to avoid problems inside the circulationloop.

The hydrocarbon heat steam generation system is a replacement to thecurrent OTSGs de-oiling and water treatment facilities, which areotherwise essential to prevent rapid fouling and tube corrosion thatoccurs in either drum boilers or OTSG systems. Use of the oil anddesalting of the oil mitigates contaminant concentration buildup in theoil and fouling within the steam generation system.

The hot hydrocarbon may give up some lighter molecular weight elementsto the steam, thus providing a small amount of solvent, and essentiallyconverting the SAGD process to an ES-SAGD process, which may reducesteam usage since the solvent has the effect of diluting and thinningthe heavy oil or bitumen. Typically, C1-C5 hydrocarbons, and even C6-C8hydrocarbons, may vaporize and be carried along with the steam, albeitin low amounts.

The invention produces high pressure steam or steam-plus-solvent whichcan be used in a SAGD reservoir or in other steam stimulation processes,such as cyclic steam generation (CSS) or steam drive (SD) also calledsteam flooding, and combinations and variations thereof.

Of course, the hot hydrocarbon picks up the dissolved solids and anyentrained oil in the dirty feedwater, but the oils are not a problem,and the dissolved solids (which may no longer be dissolved) can beremoved in a cleaning loop using known technology. Treatment units caninclude one or more of a variety of treatment units, including e.g., afilter, coalescer, desalter, dehydrator, visbreaker or electrostaticseparator.

Salts in crude oil feedstocks can cause severe problems downstream,including corrosion by acids formed by chloride salt decomposition infractionator overhead equipment, fouling of heat exchangers by saltdeposition, and poisoning of catalysts in down-stream units. Therefore,crude is typically desalted before being charged to the distillationtrain. Crude can also contain suspended solids, such as sand, clay, andiron oxide particles.

The two most typical methods of crude-oil desalting, chemical andelectrostatic separation, use hot water as the extraction agent. Inchemical desalting, water and chemical surfactant (demulsifiers) areadded to the crude, heated so that salts and other impurities dissolveinto the water or attach to the water, and then held in a tank wherethey settle out. Electrical desalting is the application of high-voltageelectrostatic charges to concentrate suspended water globules in thebottom of the settling tank. Surfactants are added only when the crudehas a large amount of suspended solids. Both methods of desalting arecontinuous. A third and less-common process involves filtering heatedcrude using diatomaceous earth.

For example, an electrostatic dehydration system is an efficient methodto remove high salinity formation water from the crude oil stream. Thisprocess relies on establishing a high voltage AC electrical field in theoil phase of dehydrator/desalter vessels. The electrical field imposesan electrical charge on water droplets entrained in the oil stream, thuscausing them to oscillate as they pass through the electrodes. Duringthis oscillation the droplets are stretched or elongated and thencontracted during reversal of the imposing AC electrical field. Duringthis agitation, the water droplets co-mingle and coalesce into dropletsof sufficient size to migrate, by gravity, back into the lower waterphase of the vessel for disposal.

Alternatively, Ultrafiltration (UF) can be used primarily to remove theemulsified oil droplets, followed by the removal of total dissolvedsolids (TDS) via reverse osmosis (RO).

The liquid boiler system described herein improves SAGD economics by:

-   -   Eliminating the need for de-oiling, water pre-treatment plants        and conventional steam boiler plants.    -   Enhancing the heavy oil recovery by including lower molecular        weight hydrocarbons combined with the produced steam. These        hydrocarbons aid in reducing the heavy oil viscosity in the        reservoir along with the steam, thus, enhancing oil production.    -   Overall SAGD steam demand may also decrease due to the presence        of hydrocarbon within the steam, in much the same manner that        ES-SAGD reduces steam requirements.

The invention includes one or more of the following embodiments, in anycombination thereof:

A steam generator system for heavy oil production, comprising: a vesselcomprising a hot hydrocarbon; a pump for pressurizing a dirty feedwaterstream fluidly connected to nozzles in said vessel, said nozzlesspraying said dirty feedwater onto said hot hydrocarbon; and an exitport near a top of said vessel for collecting pressurized steam andtransporting said pressurized steam to a wellhead injection system forinjecting steam into an oil reservoir; wherein these elements arefluidly connected.

A closed heat transfer fluid circulation loop that passes in partthrough said vessel can be used to heat said hot hydrocarbon. The closedheat transfer fluid circulation loop can comprise a heat transfer fluid,a heater, and a pump, circulating through closed coils which pass, inpart, through the liquid boiler vessel.

The liquid boiler vessel can also comprise a hot hydrocarbon treatmentloop in fluid connection with said vessel, wherein said hot hydrocarbontreatment loop either clean or upgrades the hot hydrocarbons. Exemplarytreatments include filtering, desalting, dehydrating, coalescing,visbreaking, electrostatic separating, and the like.

A liquid steam generator, comprising a vessel comprising a hothydrocarbon in a lower portion of said vessel; a closed heat transferfluid circulation loop containing a heat transfer fluid, said looppassing in part through said lower half of said vessel to heat said hothydrocarbon, the remainder passing to a heater and a pump to heat andcirculate said heat transfer fluid; a hot hydrocarbon treatment loop forcleaning said hot hydrocarbon, said hot hydrocarbon treatment loopincluding a pump and a desalter; a pump for pressurizing a dirtyfeedwater stream fluidly connected to nozzles in an upper portion ofsaid vessel, said nozzles spraying said dirty feedwater onto said hothydrocarbon; and an exit port near a top of said vessel for collectingpressurized steam and transporting said pressurized steam to a wellheadinjection system for injecting steam into an oil reservoir; wherein theelements (except for the closed circulation loop) are fluidly connected.

Exemplary hydrocarbon heat transfer fluids are selected from butane,molten sodium, molten sodium-potassium, DOWTHERM or THERMINOL.

The dirty feedwater can be any water that is not pretreated before use,including produced water, brackish water, well water, brine, surfacewater and combinations thereof. The dirty feedwater may be producedwater originating from any convenient source.

The hot hydrocarbon fluid can be any conveniently available hothydrocarbon, especially being a produced hydrocarbon separated from saidproduced water, or a fraction thereof.

The liquid boiler can produce a pressurized steam that is a mixture ofsteam and low molecular weight hydrocarbons, such as butane, pentane,and the like.

One embodiment is an improved method of steam assisted gravity drainage(SAGD), the method comprising pretreating produced water for a steamgenerator to remove oil and salts, making pressurized steam from saidpretreated water, pumping said pressurized steam into a wellbore in anamount sufficient to mobilize heavy oil, and gravity draining saidmobilized heavy oil to a production well, the improvement comprisingspray injecting pressurized dirty water into a vessel containing a hotheavy oil and collecting pressurized steam for use in SAGD, without saidwater pretreating step.

Another improved method of steam production for the mobilization ofheavy oil, the method comprising pretreating produced water for a steamgenerator to remove oil and salts, making pressurized steam from saidpretreated water, pumping said pressurized steam into a wellbore in anamount sufficient to mobilize heavy oil, and producing said mobilizedheavy oil, the improvement comprising spray injecting pressurized dirtywater into a vessel containing a hot hydrocarbon and collectingpressurized steam for use in mobilizing heavy oil, without said waterpretreating step, wherein said hot hydrocarbon is heated with a closedcirculation loop comprising a pump and a furnace to circulate a heattransfer fluid through said closed circulation loop.

By “dirty water” what is meant is that the water can be recycled fromoil recovery processes and used as is, without expensive de-oiling ordesalting pre-treatments applied to it.

The use of the word “a” or “an” when used in conjunction with the term“comprising” in the claims or the specification means one or more thanone, unless the context dictates otherwise.

The term “about” means the stated value plus or minus the margin oferror of measurement or plus or minus 10% if no method of measurement isindicated.

The use of the term “or” in the claims is used to mean “and/or” unlessexplicitly indicated to refer to alternatives only or if thealternatives are mutually exclusive.

The terms “comprise”, “have”, “include” and “contain” (and theirvariants) are open-ended linking verbs and allow the addition of otherelements when used in a claim.

The phrase “consisting of” is closed, and excludes all additionalelements.

The phrase “consisting essentially of” excludes additional materialelements, but allows the inclusions of non-material elements that do notsubstantially change the nature of the invention.

The following abbreviations are used herein:

ABBREVIATION TERM ATM Atmosphere BFW Boiler feed-water CAPEX Capitolexpenses CPF Central processing facility CSS Cyclic steam stimulationES-SAGD Expanding solvent SAGD OPEX Operating expenses OTSG Once-throughsteam generator RO Reverse osmosis SAGD Steam-assisted gravity drainageSD Steam drive TDS total dissolved solids Ts Saturation temperature UFUltrafiltration

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a highly simplified view of a modern OTSG system andused for SAGD steam production.

FIG. 2 presents a simplified drum boiler system.

FIG. 3 illustrates a simplified schematic of the liquid boiler system ofthe invention, which can be beneficially used with SAGD and othersteam-based enhanced oil recovery methods

FIG. 4 is a schematic of an alternative arrangement to contact a mixtureof water and oil with more of the oil that has been heated to thusvaporize the water and potentially result in visbreaking of the oil,according to one embodiment of the invention.

DETAILED DESCRIPTION

The disclosure provides a novel method for generating steam withminimized or eliminated fouling. The disclosure also provides a novelsystem for implementing the method.

In general, an improved method of generating steam for SAGD and otherheavy oil production uses is provided, wherein a hot liquid hydrocarbonis used to convert water to steam, and wherein the steam may thuscontain lower molecular weight components stripped from the hot liquidhydrocarbon.

FIG. 3 gives one example of a liquid boiler process for SAGD. As seen inthe figure, dirty feedwater 101 that is not de-oiled or pre-treated toremove dissolved solids enters the system. Pump 103 brings the dirtywater to high pressure and then it is injected via spray nozzles 105into the liquid boiler vessel 109. Since the water is pressurized thereis little fouling of the components up to this point.

Hot liquid 113 (e.g., produced heavy hydrocarbons, etc.) vaporizes thedirty boiler feed water sprayed into the vessel. The resulting producedsteam (with potentially some hydrocarbons in it) exits 111 out the topof the liquid-boiler and is sent by line 113 to the SAGD reservoir. Anydissolved solids or oil from the dirty feedwater remains with the hotliquid hydrocarbons.

The hot liquid receives its thermal energy from another heat transferfluid in a closed circulation loop 157 via heat transfer within coils155. The heat transfer fluid (such as butane, molten sodium, moltensodium-potassium, DOWTHERM or THERMINOL) within the coils receives itsheat via an external furnace 151, and in that sense the boiler is anindirect boiler, heat coming from an outside source. In someembodiments, the heat transfer fluid, such as butane, may be condensedfor pumping prior to being vaporized in the furnace 151 and circulatedthrough the coils 155 in the vessel 109. To the extent that producedhydrocarbons are used in the process, they already have a certain heat,decreasing initial heating costs. The hot hydrocarbons used to vaporizethe produced water may be treated by an external hydrocarbon treatmentunit 173, such as a desalter, to remove the accumulating contaminantsfrom the dirty feedwater.

The method allows the boiler to produce steam with non-treated (dirty)boiler feed water. This, therefore, reduces the CAPEX and OPEX costsassociated with de-oiling and water treatment plants. Using a liquidsuch as DOWTHERM or THERMINOL as the heat transfer liquid allows forconventional coil metallurgy, thus, minimizing the CAPEX for theindirect boiler, as well as minimizing any fouling of these coils.

FIG. 4 illustrates a hot hydrocarbon-based system with a steam generatorvessel 200, an injection well 201 and a production well 202 that areoperated for steam generation. A feed pump 216 pressurizes the dirtyfeedwater mixture 204 that can optionally be preheated in a furnace orheat exchanger 217 prior to introduction into the vessel 200. In someembodiments, the mixture 204 may receive pre-heat from a sales portion210 of the hydrocarbons.

Upon entry into the vessel 200, some flashing of the water in themixture 204 may occur upon expansion into relative lower pressureconditions of the vessel 200. However, most of the water in the mixture204 vaporizes upon contact with hot hydrocarbon 220 collected in thelower half of the vessel 200. The hydrocarbons 220 may be partiallyheated, if for example, produced hydrocarbons are used, and/or can befurther heated in closed circulation loop 257 consisting of furnace 251,pump 253 and heating coils 255 that pass through the hot hydrocarbon220.

A second circulation loop 222 contains a recycle pump 221 that passesthe hot hydrocarbon 220 from the vessel 200 to a treatment unit 223before returning the hot hydrocarbon 220 to the vessel 200. Treatmentunit 223 can include one or more of a variety of treatment units,including e.g., a filter, coalescer, desalter, dehydrator, visbreaker orelectrostatic separator. The desalter or other treatment unit 223removes inorganic material from the hot hydrocarbon 220. Some of the hothydrocarbon 220 exiting the desalter 223 can provide the sales portion210 of the hydrocarbons for pipeline or transport to a refinery forfurther processing.

For some embodiments, overhead from the vessel 200 passes through aseparation device 229 that may include demisters, separators,fractionators and/or particulate filters. The device 229 removesentrained liquids and/or solids 233 and/or condensable hydrocarbons 231vaporized by the hot hydrocarbon 220 or resulting from cracking of thehot hydrocarbon 220. The condensable hydrocarbons 231 may mix back intothe sales portion 210 of the hydrocarbons or have a portion mixed backfor injection into the formation as a solvent. However, it isanticipated that the overhead steam can be used as is, and that anylight hydrocarbons that may have evaporated along with the steam (e.g.,naptha), will reduce the steam oil ratio (SOR) needed to produce abarrel of oil.

Steam 230 exits the device 229 and is conveyed to the injection well201. Since separation of the mixture 204 occurs with the vessel 200,this approach eliminates need for independent de-oiling equipment.

Residence time of the hot hydrocarbon 220 in the vessel 200 may evenprovide sufficient soak time for visbreaking of the hydrocarbon 220. Avisbreaker thermally cracks large hydrocarbon molecules in the oil byheating in a furnace to reduce its viscosity and to produce smallquantities of light hydrocarbons (LPG and gasoline). The process name of“visbreaker” refers to the fact that the process reduces (i.e., breaks)the viscosity of the residual oil, and generally the process isnon-catalytic.

Alternatively, a visbreaker can be provided in the second circulationloop 222. Exemplary soaking times may range from 5 minutes to 1 hourwith the bitumen heated in the visbreaker to at least 385° C. Thecirculation loop 222 may incorporate various approaches to enhance thevisbreaking, such as radiation thermal cracking or hydrodynamiccavitation. The visbreaking lowers viscosity and density of the heavyoils or bitumen 220 and hence the sales portion 210 making the salesportion 210 more valuable and easier to transport while requiring lessdiluents than the bitumen without such upgrading.

In some embodiments, the water supplied for generation of the steam mayinclude boiler blowdown from another steam generator, such as aonce-through steam generator. The methods disclosed herein may providefor treatment of such blowdown. Further, the steam generated by suchtreatment may be at pressures lower than desired for injection and maybe recycled for mixing with boiler feed water prior to generation ofsteam for injection.

Based on the above illustrations, it is clearly shown that the methodsand systems herein described pressurize the feedwater before it entersthe heating mechanism and thereby avoids the nucleate boiling phase thatdirectly contributes to fouling. Downtime for pigging/repairing theboiler and pipes can be greatly reduced, therefore cutting down theoperation cost.

The following documents are incorporated by reference in their entirety:

Gwak et al., A Review of Steam Generation for In-Situ Oil SandsProjects, Geosystem Engineering, 13(3), 111-118 (September 2010).

What is claimed is:
 1. A steam generator system for heavy oilproduction, comprising: a vessel comprising a hydrocarbon; a closed heattransfer fluid circulation loop that passes in part through the vesselto heat the hydrocarbon; a pump for pressurizing a feedwater stream andfluidly connected to nozzles in the vessel, wherein the nozzles spraythe feedwater onto the hydrocarbon to produce steam; and a wellheadinjection system for conveying the steam into an oil reservoir andcoupled to an exit port near a top of the vessel for collecting thesteam.
 2. The steam generator system of claim 1, wherein the closed heattransfer fluid circulation loop includes a heat transfer fluid, a heaterand a pump.
 3. The steam generator system of claim 1, wherein the closedheat transfer fluid circulation loop includes a heat transfer fluidselected from butane, molten sodium, molten sodium-potassium, DOWTHERMand THERMINOL.
 4. The steam generator system of claim 1, furthercomprising a hydrocarbon treatment loop in fluid connection with thevessel, wherein the hydrocarbon treatment loop desalts the hydrocarbon.5. The steam generator system of claim 2, further comprising ahydrocarbon treatment loop in fluid connection with the vessel, whereinthe hydrocarbon treatment loop desalts the hydrocarbon.
 6. The steamgenerator system of claim 3, further comprising a hydrocarbon treatmentloop in fluid connection with the vessel, wherein the hydrocarbontreatment loop desalts the hydrocarbon.
 7. A liquid steam generator,comprising: a vessel comprising a hydrocarbon in a lower portion of thevessel; a closed heat transfer fluid circulation loop containing a heattransfer fluid, wherein the loop passes in part through the lower halfof the vessel to heat the hydrocarbon and a remainder of the loop passesto a heater and a pump to heat and circulate the heat transfer fluid; ahydrocarbon treatment loop for cleaning the hydrocarbon, wherein thehydrocarbon treatment loop includes a pump and a desalter; a pump forpressurizing a feedwater stream fluidly connected to nozzles in an upperportion of the vessel, wherein the nozzles spray the feedwater onto thehydrocarbon to produce steam; and a wellhead injection system forconveying the steam into an oil reservoir and coupled to an exit portnear a top of the vessel for collecting the steam.
 8. The steamgenerator system of claim 7, wherein the hydrocarbon heat transfer fluidis selected from butane, molten sodium, molten sodium-potassium,DOWTHERM and THERMINOL.
 9. The liquid steam generator of claim 7,wherein the feedwater is untreated produced water.
 10. The liquid steamgenerator of claim 9, wherein the hydrocarbon fluid is a producedhydrocarbon separated from the produced water.
 11. The liquid steamgenerator of claim 7, wherein a mixture of the steam and at least someof the hydrocarbons with less than eight carbon atoms per moleculeoutput the vessel through the exit port.
 12. The liquid steam generatorof claim 7, wherein the treatment loop includes a visbreaker.
 13. Amethod of generating steam, comprising: circulating a heat transferfluid through a closed loop for transfer of thermal energy from a heateralong the loop to hydrocarbons in a vessel as a portion of the looppasses through the vessel; and introducing feedwater into contact withthe hydrocarbons in the vessel to vaporize the feedwater into steam. 14.The method of claim 13, wherein the feedwater is untreated producedwater.
 15. The method of claim 13, wherein the feedwater is blowdownfrom one of a steam generator and an evaporator.
 16. The method of claim13, wherein the heat transfer fluid is selected from butane, moltensodium, molten sodium-potassium, DOWTHERM and THERMINOL.
 17. The methodof claim 13, further comprising injecting the steam into an oilreservoir.
 18. The method of claim 13, further comprising desalting thehydrocarbon in the vessel.
 19. The method of claim 13, wherein an outputfrom the vessel includes a mixture of the steam and at least some of thehydrocarbons with less than eight carbon atoms per molecule.
 20. Themethod of claim 13, further comprising circulating the hydrocarbons intocontact with the steam output from the vessel.